The TRAFO QUIZ: Why Zero Volt? Help for a newbie...

[Houseman]

Hi forum.
I have recovered from the net a new ANTENNA TV ROTOR styled 80-90 when they used to point the receiving antenna in different directions.
The fact is that it's original made in the USA so 110V AC input.
Apart from using a 230V to 110V transformer I have opened it and want to remake the trafo from an Italian manifacturer.
The Secondary has three terminals so two secondary windings.
.Black
1st Red
2nd.Red
The Black terminal is the common from both windings (I can clearly see the twist of the two windings)
Between Black and first Red I measure 21,5V
Between Black and second Red I measure 21,5V
Between 1st Red and 2nd Red I measure...... guess what? ZERO Volt!
If the black is the common point between the two windings shouldn't I read from RED to RED the sum of both so 43V?

What am I missing so basilar?

Thank You so much

[james_s]

My guess is that you are mistaken in your assumption of how the windings are arranged. Disconnect all of the wires from whatever they are connected to and with the power off measure the resistance from each wire to each of the other wires. That will tell you exactly which wires go to which winding(s) and then you can correlate that to what you measure coming out with the transformer powered.

[Paul Rose]

Maybe the 2 secondaries are connected out of phase, such that the "bottom" of one secondary is connected to the bottom of the other (instead of bottom of one to top of other).  In this case, the AC voltages would cancel instead of add.

Depending on how it was originally used, maybe they didn't need a "center tap" arrangement.

(I can clearly see the twist of the two windings)

Are the twisted common long enough that you could un-twist?

[Houseman]

Maybe the 2 secondaries are connected out of phase, such that the "bottom" of one secondary is connected to the bottom of the other (instead of bottom of one to top of other).  In this case, the AC voltages would cancel instead of add.

Depending on how it was originally used, maybe they didn't need a "center tap" arrangement.

(I can clearly see the twist of the two windings)

Are the twisted common long enough that you could un-twist?

I would not prefer to untwist it since one possibility would be to rewind the primary in order to accept 230V instead of 110.
Thank you for your answer.
Both secondary must control two motor (one is the rotor on the mast, the other is the motor inside the controller that turns the dial accordingly to the turn of the main pot.
Both must turn clockwise and counterclockwise, depending what you want,
 I was thinking that way:
I plugged an oscilloscope with one channel on each red terminal winding and ground to black.
I noticed the two signals are exactly in phase each other. Is this an artifact due to the trigger or can I rely on this saying following: if the two secondary are wound both for example clockwise in parallel, one wire next to each other, around the core, would it be possible that measuring red-red the current turns first clockwise and then after it passes the black tap it does the same path counterclockwise cancelling their magnetic flux respectively so that the net resulting voltage is zero due to Lenz law (-V) +(+V) =0V?
Thank You all.

[Paul Rose]

I plugged an oscilloscope with one channel on each red terminal winding and ground to black.
I noticed the two signals are exactly in phase each other. Is this an artifact due to the trigger or can I rely on this

If you were looking at both signals at the same time on two channels of the same scope, then you can rely on what you see as meaning that they are in phase.   The scope is triggering both channels at the same time based on one of the channels.

if the two secondary are wound both for example clockwise in parallel, one wire next to each other, around the core, would it be possible that measuring red-red the current turns first clockwise and then after it passes the black tap it does the same path counterclockwise cancelling their magnetic flux respectively so that the net resulting voltage is zero due to Lenz law (-V) +(+V) =0V?

Yes. This is exactly what I meant by "out of phase".


Attached a pair of schematics. 
Left showing secondaries so that voltages add. 
Right (like I think yours is) showing secondaries so that voltages subtract.

[Houseman]

Thank You.
Yes I realized the two windings are there in order to be able to select rotation way of the AC motor-rotor  and therefore move the mast of the antenna clockwise or counterclockwise.
There is an electrolytic capacitor I still don't what it does..
Thank You for the support